lkml.org 
[lkml]   [2010]   [Jan]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier
    * Steven Rostedt (rostedt@goodmis.org) wrote:
    > On Sat, 2010-01-09 at 21:25 -0800, Paul E. McKenney wrote:
    > > On Sat, Jan 09, 2010 at 09:12:58PM -0500, Steven Rostedt wrote:
    >
    > > > > < user space >
    > > > >
    > > > > < misses that CPU 2 is in rcu section >
    > > >
    > > >
    > > > If the TLB flush misses that CPU 2 has a threaded task, and does not
    > > > flush CPU 2s TLB, it can also risk the same type of crash.
    > >
    > > But isn't the VM's locking helping us out in that case?
    > >
    > > > > [CPU 2's ->curr update now visible]
    > > > >
    > > > > [CPU 2's rcu_read_lock() store now visible]
    > > > >
    > > > > free(obj);
    > > > >
    > > > > use_object(obj); <=== crash!
    > > > >
    > > >
    > > > Think about it. If you change a process mmap, say you updated a mmap of
    > > > a file by flushing out one page and replacing it with another. If the
    > > > above missed sending to CPU 2, then CPU 2 may still be accessing the old
    > > > page of the file, and not the new one.
    > > >
    > > > I think this may be the safe bet.
    > >
    > > You might well be correct that we can access that bitmap locklessly,
    > > but there are additional things (like the loading of the arch-specific
    > > page-table register) that are likely to be helping in the VM case, but
    > > not necessarily helping in this case.
    >
    >
    > Then perhaps the sys_membarrier() should just do a flush_tlb()? That
    > should guarantee the synchronization, right?
    >

    The way I see it, TLB can be seen as read-only elements (a local
    read-only cache) on the processors. Therefore, we don't care if they are
    in a stale state while performing the cpumask update, because the fact
    that we are executing switch_mm() means that these TLB entries are not
    being used locally anyway and will be dropped shortly. So we have the
    equivalent of a full memory barrier (load_cr3()) _after_ the cpumask
    updates.

    However, in sys_membarrier(), we also need to flush the write buffers
    present on each processor running threads which belong to our current
    process. Therefore, we would need, in addition, a smp_mb() before the
    mm cpumask modification. For x86, cpumask_clear_cpu/cpumask_set_cpu
    implies a LOCK-prefixed operation, and hence does not need any added
    barrier, but this could be different for other architectures.

    So, AFAIK, doing a flush_tlb() would not guarantee the kind of
    synchronization we are looking for because an uncommitted write buffer
    could still sit on the remote CPU when we return from sys_membarrier().

    Thanks,

    Mathieu


    --
    Mathieu Desnoyers
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


    \
     
     \ /
      Last update: 2010-01-10 17:05    [W:4.716 / U:0.180 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site