Under some unusual context-switching patterns, it is possible to end up
with multiple threads from the same mm running concurrently with
different ASIDs:
1. CPU x schedules task t with mm p containing ASID a and generation g
This task doesn't block and the CPU doesn't context switch.
So:
* per_cpu(active_asid, x) = {g,a}
* p->context.id = {g,a}
2. Some other CPU generates an ASID rollover. The global generation is
now (g + 1). CPU x is still running t, with no context switch and
so per_cpu(reserved_asid, x) = {g,a}
3. CPU y schedules task t', which shares mm p with t. The generation
mismatches, so we take the slowpath and hit the reserved ASID from
CPU x. p is then updated so that p->context.id = {g + 1,a}
4. CPU y schedules some other task u, which has an mm != p.
5. Some other CPU generates *another* CPU rollover. The global
generation is now (g + 2). CPU x is still running t, with no context
switch and so per_cpu(reserved_asid, x) = {g,a}.
6. CPU y once again schedules task t', but now *fails* to hit the
reserved ASID from CPU x because of the generation mismatch. This
results in a new ASID being allocated, despite the fact that t is
still running on CPU x with the same mm.
Consequently, TLBIs (e.g. as a result of CoW) will not be synchronised
between the two threads.
This patch fixes the problem by updating all of the matching reserved
ASIDs when we hit on the slowpath (i.e. in step 3 above). This keeps
the reserved ASIDs in-sync with the mm and avoids the problem.
Reported-by: Tony Thompson <anthony.thompson@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
0ebea80880