Messages in this thread | | | Date | Wed, 1 Nov 2023 21:40:23 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH] rxrpc_find_service_conn_rcu: use read_seqbegin() rather than read_seqbegin_or_lock() |
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In case I was not clear, I am not saying this code is buggy.
Just none of read_seqbegin_or_lock/need_seqretry/done_seqretry helpers make any sense in this code. It can use read_seqbegin/ read_seqretry and this won't change the current behaviour.
On 11/01, Oleg Nesterov wrote: > > On 11/01, David Howells wrote: > > > > Oleg Nesterov <oleg@redhat.com> wrote: > > > > > read_seqbegin_or_lock() makes no sense unless you make "seq" odd > > > after the lockless access failed. > > > > I think you're wrong. > > I think you missed the point ;) > > > write_seqlock() turns it odd. > > It changes seqcount_t->sequence but not "seq" so this doesn't matter. > > > For instance, if the read lock is taken first: > > > > sequence seq CPU 1 CPU 2 > > ======= ======= =============================== =============== > > 0 > > 0 0 seq = 0 MUST BE EVEN > > This is correct, > > > ACCORDING TO DOC > > documentation is wrong, please see > > [PATCH 1/2] seqlock: fix the wrong read_seqbegin_or_lock/need_seqretry documentation > https://lore.kernel.org/all/20231024120808.GA15382@redhat.com/ > > > 0 0 read_seqbegin_or_lock() [lockless] > > ... > > 1 0 write_seqlock() > > 1 0 need_seqretry() [seq=even; sequence!=seq: retry] > > Yes, if CPU_1 races with write_seqlock() need_seqretry() returns true, > > > 1 1 read_seqbegin_or_lock() [exclusive] > > No. "seq" is still even, so read_seqbegin_or_lock() won't do read_seqlock_excl(), > it will do > > seq = read_seqbegin(lock); > > again. > > > Note that it spins in __read_seqcount_begin() until we get an even seq, > > indicating that no write is currently in progress - at which point we can > > perform a lockless pass. > > Exactly. And this means that "seq" is always even. > > > > See thread_group_cputime() as an example, note that it does nextseq = 1 for > > > the 2nd round. > > > > That's not especially convincing. > > See also the usage of read_seqbegin_or_lock() in fs/dcache.c and fs/d_path.c. > All other users are wrong. > > Lets start from the very beginning. This code does > > int seq = 0; > do { > read_seqbegin_or_lock(service_conn_lock, &seq); > > do_something(); > > } while (need_seqretry(service_conn_lock, seq)); > > done_seqretry(service_conn_lock, seq); > > Initially seq is even (it is zero), so read_seqbegin_or_lock(&seq) does > > *seq = read_seqbegin(lock); > > and returns. Note that "seq" is still even. > > Now. If need_seqretry(seq) detects the race with write_seqlock() it returns > true but it does NOT change this "seq", it is still even. So on the next > iteration read_seqbegin_or_lock() will do > > *seq = read_seqbegin(lock); > > again, it won't take this lock for writing. And again, seq will be even. > And so on. > > And this means that the code above is equivalent to > > do { > seq = read_seqbegin(service_conn_lock); > > do_something(); > > } while (read_seqretry(service_conn_lock, seq)); > > and this is what this patch does. > > Yes this is confusing. Again, even the documentation is wrong! That is why > I am trying to remove the misuse of read_seqbegin_or_lock(), then I am going > to change the semantics of need_seqretry() to enforce the locking on the 2nd > pass. > > Oleg.
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