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ocfs2: Prevent a livelock in dlmglue

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There is possibility of a livelock in __ocfs2_cluster_lock(). If a node were
to get an ast for an upconvert request, followed immediately by a bast,
there is a small window where the fs may downconvert the lock before the
process requesting the upconvert is able to take the lock.

This patch adds a new flag to indicate that the upconvert is still in
progress and that the dc thread should not downconvert it right now.

Wengang Wang <wen.gang.wang@oracle.com> and Joel Becker
<joel.becker@oracle.com> contributed heavily to this patch.

Reported-by: David Teigland <teigland@redhat.com>
Signed-off-by: Sunil Mushran <sunil.mushran@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>
This commit is contained in:
Sunil Mushran 2010-01-21 10:50:03 -08:00 committed by Joel Becker
parent 0b94a909eb
commit a191282601
2 changed files with 50 additions and 3 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

49
fs/ocfs2/dlmglue.c
View file

@ -875,6 +875,14 @@ static inline void ocfs2_generic_handle_convert_action(struct ocfs2_lock_res *lo
lockres_or_flags(lockres, OCFS2_LOCK_NEEDS_REFRESH);
lockres->l_level = lockres->l_requested;
/*
* We set the OCFS2_LOCK_UPCONVERT_FINISHING flag before clearing
* the OCFS2_LOCK_BUSY flag to prevent the dc thread from
* downconverting the lock before the upconvert has fully completed.
*/
lockres_or_flags(lockres, OCFS2_LOCK_UPCONVERT_FINISHING);
lockres_clear_flags(lockres, OCFS2_LOCK_BUSY);
mlog_exit_void();
@ -1134,6 +1142,7 @@ static inline void ocfs2_recover_from_dlm_error(struct ocfs2_lock_res *lockres,
mlog_entry_void();
spin_lock_irqsave(&lockres->l_lock, flags);
lockres_clear_flags(lockres, OCFS2_LOCK_BUSY);
lockres_clear_flags(lockres, OCFS2_LOCK_UPCONVERT_FINISHING);
if (convert)
lockres->l_action = OCFS2_AST_INVALID;
else
@ -1324,13 +1333,13 @@ static int __ocfs2_cluster_lock(struct ocfs2_super *osb,
again:
wait = 0;
spin_lock_irqsave(&lockres->l_lock, flags);
if (catch_signals && signal_pending(current)) {
ret = -ERESTARTSYS;
goto out;
goto unlock;
}
spin_lock_irqsave(&lockres->l_lock, flags);
mlog_bug_on_msg(lockres->l_flags & OCFS2_LOCK_FREEING,
"Cluster lock called on freeing lockres %s! flags "
"0x%lx\n", lockres->l_name, lockres->l_flags);
@ -1347,6 +1356,25 @@ again:
goto unlock;
}
if (lockres->l_flags & OCFS2_LOCK_UPCONVERT_FINISHING) {
/*
* We've upconverted. If the lock now has a level we can
* work with, we take it. If, however, the lock is not at the
* required level, we go thru the full cycle. One way this could
* happen is if a process requesting an upconvert to PR is
* closely followed by another requesting upconvert to an EX.
* If the process requesting EX lands here, we want it to
* continue attempting to upconvert and let the process
* requesting PR take the lock.
* If multiple processes request upconvert to PR, the first one
* here will take the lock. The others will have to go thru the
* OCFS2_LOCK_BLOCKED check to ensure that there is no pending
* downconvert request.
*/
if (level <= lockres->l_level)
goto update_holders;
}
if (lockres->l_flags & OCFS2_LOCK_BLOCKED &&
!ocfs2_may_continue_on_blocked_lock(lockres, level)) {
/* is the lock is currently blocked on behalf of
@ -1417,11 +1445,14 @@ again:
goto again;
}
update_holders:
/* Ok, if we get here then we're good to go. */
ocfs2_inc_holders(lockres, level);
ret = 0;
unlock:
lockres_clear_flags(lockres, OCFS2_LOCK_UPCONVERT_FINISHING);
spin_unlock_irqrestore(&lockres->l_lock, flags);
out:
/*
@ -3402,6 +3433,18 @@ recheck:
goto leave;
}
/*
* This prevents livelocks. OCFS2_LOCK_UPCONVERT_FINISHING flag is
* set when the ast is received for an upconvert just before the
* OCFS2_LOCK_BUSY flag is cleared. Now if the fs received a bast
* on the heels of the ast, we want to delay the downconvert just
* enough to allow the up requestor to do its task. Because this
* lock is in the blocked queue, the lock will be downconverted
* as soon as the requestor is done with the lock.
*/
if (lockres->l_flags & OCFS2_LOCK_UPCONVERT_FINISHING)
goto leave_requeue;
/* if we're blocking an exclusive and we have *any* holders,
* then requeue. */
if ((lockres->l_blocking == DLM_LOCK_EX)

4
fs/ocfs2/ocfs2.h
View file

@ -136,6 +136,10 @@ enum ocfs2_unlock_action {
#define OCFS2_LOCK_PENDING (0x00000400) /* This lockres is pending a
call to dlm_lock. Only
exists with BUSY set. */
#define OCFS2_LOCK_UPCONVERT_FINISHING (0x00000800) /* blocks the dc thread
* from downconverting
* before the upconvert
* has completed */
struct ocfs2_lock_res_ops;