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author | Amerigo Wang <amwang@redhat.com> | 2009-12-15 03:00:21 +0100 |
---|---|---|
committer | Linus Torvalds <torvalds@linux-foundation.org> | 2009-12-15 17:53:26 +0100 |
commit | 29671f22a8b6522db3b126a3fdfb208759ce46e3 (patch) | |
tree | c3c434fc07aae30c401fe7662b8e1399b98ffdc2 /lib | |
parent | rwsem-spinlock: remove useless function exports (diff) | |
download | linux-29671f22a8b6522db3b126a3fdfb208759ce46e3.tar.xz linux-29671f22a8b6522db3b126a3fdfb208759ce46e3.zip |
rwsem: fix rwsem_is_locked() bugs
rwsem_is_locked() tests ->activity without locks, so we should always keep
->activity consistent. However, the code in __rwsem_do_wake() breaks this
rule, it updates ->activity after _all_ readers waken up, this may give
some reader a wrong ->activity value, thus cause rwsem_is_locked() behaves
wrong.
Quote from Andrew:
"
- we have one or more processes sleeping in down_read(), waiting for access.
- we wake one or more processes up without altering ->activity
- they start to run and they do rwsem_is_locked(). This incorrectly
returns "false", because the waker process is still crunching away in
__rwsem_do_wake().
- the waker now alters ->activity, but it was too late.
"
So we need get a spinlock to protect this. And rwsem_is_locked() should
not block, thus we use spin_trylock_irqsave().
[akpm@linux-foundation.org: simplify code]
Reported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Cc: Ben Woodard <bwoodard@llnl.gov>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: WANG Cong <amwang@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'lib')
-rw-r--r-- | lib/rwsem-spinlock.c | 13 |
1 files changed, 13 insertions, 0 deletions
diff --git a/lib/rwsem-spinlock.c b/lib/rwsem-spinlock.c index 39a74110f55b..ccf95bff7984 100644 --- a/lib/rwsem-spinlock.c +++ b/lib/rwsem-spinlock.c @@ -17,6 +17,19 @@ struct rwsem_waiter { #define RWSEM_WAITING_FOR_WRITE 0x00000002 }; +int rwsem_is_locked(struct rw_semaphore *sem) +{ + int ret = 1; + unsigned long flags; + + if (spin_trylock_irqsave(&sem->wait_lock, flags)) { + ret = (sem->activity != 0); + spin_unlock_irqrestore(&sem->wait_lock, flags); + } + return ret; +} +EXPORT_SYMBOL(rwsem_is_locked); + /* * initialise the semaphore */ |