On 14/09/2017 10:13, Sergey Senozhatsky wrote: > Hi, > > On (09/14/17 09:55), Laurent Dufour wrote: > [..] >>> so if there are two CPUs, one doing write_seqcount() and the other one >>> doing read_seqcount() then what can happen is something like this >>> >>> CPU0 CPU1 >>> >>> fs_reclaim_acquire() >>> write_seqcount_begin() >>> fs_reclaim_acquire() read_seqcount_begin() >>> write_seqcount_end() >>> >>> CPU0 can't write_seqcount_end() because of fs_reclaim_acquire() from >>> CPU1, CPU1 can't read_seqcount_begin() because CPU0 did write_seqcount_begin() >>> and now waits for fs_reclaim_acquire(). makes sense? >> >> Yes, this makes sense. >> >> But in the case of this series, there is no call to >> __read_seqcount_begin(), and the reader (the speculative page fault >> handler), is just checking for (vm_seq & 1) and if this is true, simply >> exit the speculative path without waiting. >> So there is no deadlock possibility. > > probably lockdep just knows that those locks interleave at some > point. > > > by the way, I think there is one path that can spin > > find_vma_srcu() > read_seqbegin() > read_seqcount_begin() > raw_read_seqcount_begin() > __read_seqcount_begin() That's right, but here this is the sequence counter mm->mm_seq, not the vm_seq one. This one is held to protect walking the VMA list "locklessly"... Cheers, Laurent. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
