rseq: uapi: Update uapi comments
Update rseq uapi header comments to reflect that user-space need to do thread-local loads/stores from/to the struct rseq fields. As a consequence of this added requirement, the kernel does not need to perform loads/stores with single-copy atomicity. Update the comment associated to the "flags" fields to describe more accurately that it's only useful to facilitate single-stepping through rseq critical sections with debuggers. Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: linux-api@vger.kernel.org Cc: Peter Zijlstra <peterz@infradead.org> Cc: "Paul E . McKenney" <paulmck@linux.vnet.ibm.com> Cc: Boqun Feng <boqun.feng@gmail.com> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Dave Watson <davejwatson@fb.com> Cc: Paul Turner <pjt@google.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Russell King <linux@arm.linux.org.uk> Cc: "H . Peter Anvin" <hpa@zytor.com> Cc: Andi Kleen <andi@firstfloor.org> Cc: Chris Lameter <cl@linux.com> Cc: Ben Maurer <bmaurer@fb.com> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Josh Triplett <josh@joshtriplett.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Will Deacon <will.deacon@arm.com> Cc: Michael Kerrisk <mtk.manpages@gmail.com> Cc: Joel Fernandes <joelaf@google.com> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Cc: "H. Peter Anvin" <hpa@zytor.com> Link: https://lkml.kernel.org/r/20180709195155.7654-4-mathieu.desnoyers@efficios.com
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@ -67,28 +67,30 @@ struct rseq_cs {
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struct rseq {
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/*
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* Restartable sequences cpu_id_start field. Updated by the
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* kernel, and read by user-space with single-copy atomicity
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* semantics. Aligned on 32-bit. Always contains a value in the
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* range of possible CPUs, although the value may not be the
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* actual current CPU (e.g. if rseq is not initialized). This
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* CPU number value should always be compared against the value
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* of the cpu_id field before performing a rseq commit or
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* returning a value read from a data structure indexed using
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* the cpu_id_start value.
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* kernel. Read by user-space with single-copy atomicity
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* semantics. This field should only be read by the thread which
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* registered this data structure. Aligned on 32-bit. Always
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* contains a value in the range of possible CPUs, although the
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* value may not be the actual current CPU (e.g. if rseq is not
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* initialized). This CPU number value should always be compared
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* against the value of the cpu_id field before performing a rseq
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* commit or returning a value read from a data structure indexed
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* using the cpu_id_start value.
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*/
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__u32 cpu_id_start;
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/*
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* Restartable sequences cpu_id field. Updated by the kernel,
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* and read by user-space with single-copy atomicity semantics.
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* Aligned on 32-bit. Values RSEQ_CPU_ID_UNINITIALIZED and
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* RSEQ_CPU_ID_REGISTRATION_FAILED have a special semantic: the
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* former means "rseq uninitialized", and latter means "rseq
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* initialization failed". This value is meant to be read within
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* rseq critical sections and compared with the cpu_id_start
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* value previously read, before performing the commit instruction,
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* or read and compared with the cpu_id_start value before returning
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* a value loaded from a data structure indexed using the
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* cpu_id_start value.
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* Restartable sequences cpu_id field. Updated by the kernel.
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* Read by user-space with single-copy atomicity semantics. This
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* field should only be read by the thread which registered this
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* data structure. Aligned on 32-bit. Values
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* RSEQ_CPU_ID_UNINITIALIZED and RSEQ_CPU_ID_REGISTRATION_FAILED
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* have a special semantic: the former means "rseq uninitialized",
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* and latter means "rseq initialization failed". This value is
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* meant to be read within rseq critical sections and compared
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* with the cpu_id_start value previously read, before performing
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* the commit instruction, or read and compared with the
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* cpu_id_start value before returning a value loaded from a data
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* structure indexed using the cpu_id_start value.
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*/
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__u32 cpu_id;
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/*
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@ -105,27 +107,28 @@ struct rseq {
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* targeted by the rseq_cs. Also needs to be set to NULL by user-space
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* before reclaiming memory that contains the targeted struct rseq_cs.
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*
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* Read and set by the kernel with single-copy atomicity semantics.
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* Set by user-space with single-copy atomicity semantics. Aligned
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* on 64-bit.
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* Read and set by the kernel. Set by user-space with single-copy
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* atomicity semantics. This field should only be updated by the
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* thread which registered this data structure. Aligned on 64-bit.
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*/
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LINUX_FIELD_u32_u64(rseq_cs);
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/*
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* - RSEQ_DISABLE flag:
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* Restartable sequences flags field.
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*
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* This field should only be updated by the thread which
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* registered this data structure. Read by the kernel.
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* Mainly used for single-stepping through rseq critical sections
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* with debuggers.
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*
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* Fallback fast-track flag for single-stepping.
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* Set by user-space if lack of progress is detected.
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* Cleared by user-space after rseq finish.
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* Read by the kernel.
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* - RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT
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* Inhibit instruction sequence block restart and event
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* counter increment on preemption for this thread.
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* Inhibit instruction sequence block restart on preemption
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* for this thread.
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* - RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL
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* Inhibit instruction sequence block restart and event
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* counter increment on signal delivery for this thread.
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* Inhibit instruction sequence block restart on signal
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* delivery for this thread.
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* - RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE
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* Inhibit instruction sequence block restart and event
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* counter increment on migration for this thread.
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* Inhibit instruction sequence block restart on migration for
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* this thread.
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*/
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__u32 flags;
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} __attribute__((aligned(4 * sizeof(__u64))));
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@ -201,7 +201,7 @@ static int clear_rseq_cs(struct task_struct *t)
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* of code outside of the rseq assembly block. This performs
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* a lazy clear of the rseq_cs field.
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*
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* Set rseq_cs to NULL with single-copy atomicity.
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* Set rseq_cs to NULL.
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*/
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return put_user(0UL, &t->rseq->rseq_cs);
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}
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