Fw: Re: [PATCH] bsdacct: delete timer with sync intension
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From: Vitaliy Gusev on 5 May 2010 19:00 Hi, Andrew! 27.04.10, 15:54, "Andrew Morton" <akpm(a)linux-foundation.org>: > On Thu, 25 Mar 2010 17:35:10 +0300 > Vitaliy Gusev wrote: > > > acct_exit_ns --> acct_file_reopen deletes timer without > > check timer execution on other CPUs. So acct_timeout() can > > change an unmapped memory. > > > > That sounds ugly. > > > > > --- > > kernel/acct.c | 17 +++++++++-------- > > 1 files changed, 9 insertions(+), 8 deletions(-) > > > > diff --git a/kernel/acct.c b/kernel/acct.c > > index a6605ca..6ac80ca 100644 > > --- a/kernel/acct.c > > +++ b/kernel/acct.c > > @@ -353,17 +353,18 @@ restart: > > > > void acct_exit_ns(struct pid_namespace *ns) > > { > > - struct bsd_acct_struct *acct; > > + struct bsd_acct_struct *acct = ns->bacct; > > > > - spin_lock(&acct_lock); > > - acct = ns->bacct; > > - if (acct != NULL) { > > - if (acct->file != NULL) > > - acct_file_reopen(acct, NULL, NULL); > > + if (acct == NULL) > > + return; > > > > - kfree(acct); > > - } > > + del_timer_sync(&acct->timer); > > + spin_lock(&acct_lock); > > + if (acct->file != NULL) > > + acct_file_reopen(acct, NULL, NULL); > > spin_unlock(&acct_lock); > > + > > + kfree(acct); > > } > > > > Is this sufficient? acct_file_reopen() does a del_timer(), so > acct_timeout() could be running concurrently with acct_file_reopen(), > but acct_file_reopen() is merrily altering data at *acct. Yes, It is sufficient. Don't mind about concurency acct_file_reopen() with acct_timeout(). It is safe. Even if acct_timeout occurs after del_timer, then only one bad thing can be - set needcheck at valid *acct. > > Perhaps acct_file_reopen() should be using del_timer_sync()? acct_file_reopen() is called within locked &acct_lock, and unlock/lock will bring another race. > > check_free_space() is doing a similar thing: > > del_timer(&acct->timer); > acct->needcheck = 0; > > the currently-running timer handler now goes and sets needcheck again! check_free_space() is called only for active task in pid_namespace. But acct_exit_ns() is called when there is no any thread in pid_namespace. Thus timer handler will no set again. > > Methinks the whole thing needs a bit of a rethink, bearing in mind how > del_timer() actually works. > -- Vitaliy Gusev -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |