[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH v3 05/15] seqlock: use atomic writes for the seq
From: |
Alex Bennée |
Subject: |
Re: [Qemu-devel] [PATCH v3 05/15] seqlock: use atomic writes for the sequence |
Date: |
Fri, 30 Sep 2016 23:45:19 +0100 |
User-agent: |
mu4e 0.9.17; emacs 25.1.50.2 |
Jonathan Neuschäfer <address@hidden> writes:
> On Fri, Sep 30, 2016 at 10:30:56PM +0100, Alex Bennée wrote:
>> From: Paolo Bonzini <address@hidden>
>>
>> There is a data race if the sequence is written concurrently to the
>> read. In C11 this has undefined behavior. Use atomic_set; the
>> read side is already using atomic_read.
>>
>> Reported-by: Alex Bennée <address@hidden>
>> Signed-off-by: Paolo Bonzini <address@hidden>
>> Signed-off-by: Alex Bennée <address@hidden>
>> ---
>> include/qemu/seqlock.h | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/qemu/seqlock.h b/include/qemu/seqlock.h
>> index 2e2be4c..8dee11d 100644
>> --- a/include/qemu/seqlock.h
>> +++ b/include/qemu/seqlock.h
>> @@ -31,7 +31,7 @@ static inline void seqlock_init(QemuSeqLock *sl)
>> /* Lock out other writers and update the count. */
>> static inline void seqlock_write_begin(QemuSeqLock *sl)
>> {
>> - ++sl->sequence;
>> + atomic_set(&sl->sequence, sl->sequence + 1);
>
> I'm not an atomics expert, but as I read the code, the load of
> sl->sequence (on the right side) isn't atomic relative to the store
> (atomic_set). Should this be atomic_inc(&sl->sequence) instead, or am I
> missing something?
There can only be one seqlock_write going on at a time (that is
protected by a lock). The atomic_set only ensures that the seqlock_read
side unambiguously sees one value or the other - you don't need to use
an atomic_inc in this case.
>
> In particular, I'm worried about this situation:
>
> thread 0 thread 1
> seqlock_write_begin: seqlock_write_begin:
> unsigned tmp = sl->sequence unsigned tmp = sl->sequence
> tmp += 1 tmp += 1
> atomic_set(&sl->sequence, tmp)
> atomic_set(&sl->sequence, tmp)
>
> ... where sl->sequence will effectively only be incremented once (as far
> as I can see).
>
>
> Regards,
> Jonathan Neuschäfer
--
Alex Bennée
- [Qemu-devel] [PATCH v3 00/15] A number of fixes for ThreadSanitizer, Alex Bennée, 2016/09/30
- [Qemu-devel] [PATCH v3 01/15] atomic.h: fix __SANITIZE_THREAD__ build, Alex Bennée, 2016/09/30
- [Qemu-devel] [PATCH v3 03/15] exec-all.h: revert tb_page_addr_t to target_ulong, Alex Bennée, 2016/09/30
- [Qemu-devel] [PATCH v3 06/15] qom/object: update class cache atomically, Alex Bennée, 2016/09/30
- [Qemu-devel] [PATCH v3 02/15] atomic.h: comment on use of atomic_read/set, Alex Bennée, 2016/09/30
- [Qemu-devel] [PATCH v3 04/15] tcg/optimize: move default return out of if statement, Alex Bennée, 2016/09/30
- [Qemu-devel] [PATCH v3 10/15] linux-user/syscall: extend lock around cpu-list, Alex Bennée, 2016/09/30
- [Qemu-devel] [PATCH v3 05/15] seqlock: use atomic writes for the sequence, Alex Bennée, 2016/09/30
- [Qemu-devel] [PATCH v3 08/15] cpu: atomically modify cpu->exit_request, Alex Bennée, 2016/09/30
- [Qemu-devel] [PATCH v3 07/15] qom/cpu: atomically clear the tb_jmp_cache, Alex Bennée, 2016/09/30
- [Qemu-devel] [PATCH v3 11/15] qga/command: use QEMU atomic primitives, Alex Bennée, 2016/09/30
- [Qemu-devel] [PATCH v3 12/15] .travis.yml: add gcc sanitizer build, Alex Bennée, 2016/09/30
- [Qemu-devel] [PATCH v3 13/15] tcg: ensure cpu_tb_exec/tb_gen_code use atomic_read/write, Alex Bennée, 2016/09/30
- [Qemu-devel] [PATCH v3 15/15] translate-all: mark updates to PageDesc as atomic, Alex Bennée, 2016/09/30
- [Qemu-devel] [PATCH v3 09/15] util/qht: atomically set b->hashes, Alex Bennée, 2016/09/30
- [Qemu-devel] [PATCH v3 14/15] tcg: update remaining TranslationBuffer fields atomically, Alex Bennée, 2016/09/30