On 1/19/22 23:02, Vladimir Kondratyev wrote:
On 19.01.2022 12:50, Hans Petter Selasky wrote:
On 1/18/22 22:35, Vladimir Kondratyev wrote:
Any ideas how to avoid it in a generic way?
Hi,
On a non-SMP system this will lead to deadlock.
Is it possible you can pre-lock this spin lock earlier, so that it is
already locked, so instead of
while(trylock());
You have:
assert (trylock() XXX)
Unfortunately, vulnerable functions are called in too many code paths to
patch them all.
Or else,
convert this particular lock to a native FreeBSD spinlock mutex.
It can be done for wake_up() but not for dma_fence_signal() which
suffers from this problem too. Some code that uses that lock expect it
to be spinlock_t
I think we can just drop critical section in seqlock-related part of
dma-buf code and replace it with rwlock as kib@ and mjg@ suggested.
Leave seqlock for actual locking to preserve semantics as much as
possible and add rwlock to implement reader's blocking. Following
snippets show code conversion required for this change:
Lock seqlock as reader:
retry:
seq = read_seqcount_begin(&obj->seq);
... reader payload ...
if (read_seqcount_retry(&obj->seq, seq)) {
#ifdef __FreeBSD__
/* Wait for rwlock to be released by writer */
rw_rlock(&obj->rwlock);
rw_runlock(&obj->rwlock);
#endif
goto retry;
}
Lock seqlock as writer:
#ifdef __linux__
preempt_disable();
#elif defined (__FreeBSD__)
rw_wlock(&obj->rwlock);
#endif
write_seqcount_begin(&obj->seq);
... writer payload ...
write_seqcount_end(&obj->seq);
#ifdef __linux__
preempt_enable();
#elif defined (__FreeBSD__)
rw_wunlock(&obj->rwlock);
#endif
Hi Vladimir,
Waiting for a differential revision.
--HPS
