From: Peng Zheng <pengzheng@apache.org>
To: libc-alpha@sourceware.org
Cc: triegel@redhat.com
Subject: Re: Priority Inversion and Unlimited Spin of pthread_rwlock_t
Date: Thu, 14 Mar 2024 15:32:26 +0800 [thread overview]
Message-ID: <ffd9db52-9a1b-4ade-9d75-419eab56b91f@apache.org> (raw)
In-Reply-To: <b68aa613-7a1f-4fae-8bb9-f306b76cba74@apache.org>
On 2024/3/12 11:48, Peng Zheng wrote:
> On 2024/3/12 11:19, Peng Zheng wrote:
>> Hi,
>>
>> I found that there are several unlimited spins in the current
>> pthread_rwlock's implementation.
>> Therefore, it suffers from the same issue of user-space spinlocks as
>> mentioned in this LWN article ([0]):
>
> I forget to mention this issue is glibc-specific.
> Just checked musl's source code, all spin in its pthread_rwlock_t is
> limited to 100. For example:
It seems that this issue is introduced by
cc25c8b4c1196a8c29e9a45b1e096b99a87b7f8c, whose design is quite
complicated.
It's highly non-trivial to add limited spin to the current implementation.
This issue has extensive impact because OpenSSL use wrlock as ordinary
mutex by default.
https://github.com/openssl/openssl/blob/3cb0755323281267211fbe951b94a2552e99d32a/crypto/threads_pthread.c#L622
And it impacts nearly all subsystems of OpenSSL.
https://github.com/openssl/openssl/blob/b372b1f76450acdfed1e2301a39810146e28b02c/crypto/ex_data.c#L50C22-L50C34
I can safely tell that nearly all linux running real-time task using
OpenSSL might be affected by this issue (like real-time video streaming
over TLS connection).
>
> int __pthread_rwlock_timedwrlock(pthread_rwlock_t *restrict rw, const
> struct timespec *restrict at)
> {
> int r, t;
>
> r = pthread_rwlock_trywrlock(rw);
> if (r != EBUSY) return r;
>
> int spins = 100;
> while (spins-- && rw->_rw_lock && !rw->_rw_waiters) a_spin();
>
> while ((r=__pthread_rwlock_trywrlock(rw))==EBUSY) {
> if (!(r=rw->_rw_lock)) continue;
> t = r | 0x80000000;
> a_inc(&rw->_rw_waiters);
> a_cas(&rw->_rw_lock, r, t);
> r = __timedwait(&rw->_rw_lock, t, CLOCK_REALTIME, at,
> rw->_rw_shared^128);
> a_dec(&rw->_rw_waiters);
> if (r && r != EINTR) return r;
> }
> return r;
> }
>
> Regards,
>
--
Peng Zheng
next prev parent reply other threads:[~2024-03-14 7:32 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-12 3:19 Peng Zheng
2024-03-12 3:48 ` Peng Zheng
2024-03-14 7:32 ` Peng Zheng [this message]
2024-03-12 14:39 ` Florian Weimer
2024-03-13 1:48 ` Peng Zheng
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