Re: [PATCH 1/2] freezer: add unsafe versions of freezable helpers

[Jeff Layton]

On Mon, 6 May 2013 14:54:53 -0700
Colin Cross <ccross@xxxxxxxxxxx> wrote:

> On Mon, May 6, 2013 at 2:43 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> > On Mon, 6 May 2013 12:57:54 -0700
> > Colin Cross <ccross@xxxxxxxxxxx> wrote:
> >
> >> On Mon, May 6, 2013 at 3:56 AM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> >> > On Fri,  3 May 2013 14:04:09 -0700
> >> > Colin Cross <ccross@xxxxxxxxxxx> wrote:
> >> >
> >> >> NFS calls the freezable helpers with locks held, which is unsafe
> >> >> and caused lockdep warnings when 6aa9707 "lockdep: check that no
> >> >> locks held at freeze time" was applied (reverted in dbf520a).
> >> >> Add new *_unsafe versions of the helpers that will not run the
> >> >> lockdep test when 6aa9707 is reapplied, and call them from NFS.
> >> >>
> >> >> Signed-off-by: Colin Cross <ccross@xxxxxxxxxxx>
> >> >> ---
> >> >>  fs/nfs/inode.c          |  2 +-
> >> >>  fs/nfs/nfs3proc.c       |  2 +-
> >> >>  fs/nfs/nfs4proc.c       |  4 ++--
> >> >>  include/linux/freezer.h | 42 +++++++++++++++++++++++++++++++++++++++++-
> >> >>  net/sunrpc/sched.c      |  2 +-
> >> >>  5 files changed, 46 insertions(+), 6 deletions(-)
> >> >>
> >> >> diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
> >> >> index 1f94167..53cbee5 100644
> >> >> --- a/fs/nfs/inode.c
> >> >> +++ b/fs/nfs/inode.c
> >> >> @@ -79,7 +79,7 @@ int nfs_wait_bit_killable(void *word)
> >> >>  {
> >> >>       if (fatal_signal_pending(current))
> >> >>               return -ERESTARTSYS;
> >> >> -     freezable_schedule();
> >> >> +     freezable_schedule_unsafe();
> >> >>       return 0;
> >> >>  }
> >> >>  EXPORT_SYMBOL_GPL(nfs_wait_bit_killable);
> >> >> diff --git a/fs/nfs/nfs3proc.c b/fs/nfs/nfs3proc.c
> >> >> index 43ea96c..ce90eb4 100644
> >> >> --- a/fs/nfs/nfs3proc.c
> >> >> +++ b/fs/nfs/nfs3proc.c
> >> >> @@ -33,7 +33,7 @@ nfs3_rpc_wrapper(struct rpc_clnt *clnt, struct rpc_message *msg, int flags)
> >> >>               res = rpc_call_sync(clnt, msg, flags);
> >> >>               if (res != -EJUKEBOX)
> >> >>                       break;
> >> >> -             freezable_schedule_timeout_killable(NFS_JUKEBOX_RETRY_TIME);
> >> >> +             freezable_schedule_timeout_killable_unsafe(NFS_JUKEBOX_RETRY_TIME);
> >> >>               res = -ERESTARTSYS;
> >> >>       } while (!fatal_signal_pending(current));
> >> >>       return res;
> >> >> diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
> >> >> index 0ad025e..a236077 100644
> >> >> --- a/fs/nfs/nfs4proc.c
> >> >> +++ b/fs/nfs/nfs4proc.c
> >> >> @@ -266,7 +266,7 @@ static int nfs4_delay(struct rpc_clnt *clnt, long *timeout)
> >> >>               *timeout = NFS4_POLL_RETRY_MIN;
> >> >>       if (*timeout > NFS4_POLL_RETRY_MAX)
> >> >>               *timeout = NFS4_POLL_RETRY_MAX;
> >> >> -     freezable_schedule_timeout_killable(*timeout);
> >> >> +     freezable_schedule_timeout_killable_unsafe(*timeout);
> >> >>       if (fatal_signal_pending(current))
> >> >>               res = -ERESTARTSYS;
> >> >>       *timeout <<= 1;
> >> >> @@ -4309,7 +4309,7 @@ int nfs4_proc_delegreturn(struct inode *inode, struct rpc_cred *cred, const nfs4
> >> >>  static unsigned long
> >> >>  nfs4_set_lock_task_retry(unsigned long timeout)
> >> >>  {
> >> >> -     freezable_schedule_timeout_killable(timeout);
> >> >> +     freezable_schedule_timeout_killable_unsafe(timeout);
> >> >>       timeout <<= 1;
> >> >>       if (timeout > NFS4_LOCK_MAXTIMEOUT)
> >> >>               return NFS4_LOCK_MAXTIMEOUT;
> >> >> diff --git a/include/linux/freezer.h b/include/linux/freezer.h
> >> >> index e70df40..5b31e21c 100644
> >> >> --- a/include/linux/freezer.h
> >> >> +++ b/include/linux/freezer.h
> >> >> @@ -46,7 +46,11 @@ extern int freeze_kernel_threads(void);
> >> >>  extern void thaw_processes(void);
> >> >>  extern void thaw_kernel_threads(void);
> >> >>
> >> >> -static inline bool try_to_freeze(void)
> >> >> +/*
> >> >> + * DO NOT ADD ANY NEW CALLERS OF THIS FUNCTION
> >> >> + * If try_to_freeze causes a lockdep warning it means the caller may deadlock
> >> >> + */
> >> >> +static inline bool try_to_freeze_unsafe(void)
> >> >>  {
> >> >>       might_sleep();
> >> >>       if (likely(!freezing(current)))
> >> >> @@ -54,6 +58,11 @@ static inline bool try_to_freeze(void)
> >> >>       return __refrigerator(false);
> >> >>  }
> >> >>
> >> >> +static inline bool try_to_freeze(void)
> >> >> +{
> >> >> +     return try_to_freeze_unsafe();
> >> >> +}
> >> >> +
> >> >>  extern bool freeze_task(struct task_struct *p);
> >> >>  extern bool set_freezable(void);
> >> >>
> >> >> @@ -115,6 +124,14 @@ static inline void freezer_count(void)
> >> >>       try_to_freeze();
> >> >>  }
> >> >>
> >> >> +/* DO NOT ADD ANY NEW CALLERS OF THIS FUNCTION */
> >> >> +static inline void freezer_count_unsafe(void)
> >> >> +{
> >> >> +     current->flags &= ~PF_FREEZER_SKIP;
> >> >> +     smp_mb();
> >> >> +     try_to_freeze_unsafe();
> >> >> +}
> >> >> +
> >> >>  /**
> >> >>   * freezer_should_skip - whether to skip a task when determining frozen
> >> >>   *                    state is reached
> >> >> @@ -152,6 +169,14 @@ static inline bool freezer_should_skip(struct task_struct *p)
> >> >>       freezer_count();                                                \
> >> >>  })
> >> >>
> >> >> +/* DO NOT ADD ANY NEW CALLERS OF THIS FUNCTION */
> >> >> +#define freezable_schedule_unsafe()                                  \
> >> >> +({                                                                   \
> >> >> +     freezer_do_not_count();                                         \
> >> >> +     schedule();                                                     \
> >> >> +     freezer_count_unsafe();                                         \
> >> >> +})
> >> >> +
> >> >>  /* Like schedule_timeout_killable(), but should not block the freezer. */
> >> >>  #define freezable_schedule_timeout_killable(timeout)                 \
> >> >>  ({                                                                   \
> >> >> @@ -162,6 +187,16 @@ static inline bool freezer_should_skip(struct task_struct *p)
> >> >>       __retval;                                                       \
> >> >>  })
> >> >>
> >> >> +/* DO NOT ADD ANY NEW CALLERS OF THIS FUNCTION */
> >> >> +#define freezable_schedule_timeout_killable_unsafe(timeout)          \
> >> >> +({                                                                   \
> >> >> +     long __retval;                                                  \
> >> >> +     freezer_do_not_count();                                         \
> >> >> +     __retval = schedule_timeout_killable(timeout);                  \
> >> >> +     freezer_count_unsafe();                                         \
> >> >> +     __retval;                                                       \
> >> >> +})
> >> >> +
> >> >>  /*
> >> >>   * Freezer-friendly wrappers around wait_event_interruptible(),
> >> >>   * wait_event_killable() and wait_event_interruptible_timeout(), originally
> >> >> @@ -225,9 +260,14 @@ static inline void set_freezable(void) {}
> >> >>
> >> >>  #define freezable_schedule()  schedule()
> >> >>
> >> >> +#define freezable_schedule_unsafe()  schedule()
> >> >> +
> >> >>  #define freezable_schedule_timeout_killable(timeout)                 \
> >> >>       schedule_timeout_killable(timeout)
> >> >>
> >> >> +#define freezable_schedule_timeout_killable_unsafe(timeout)          \
> >> >> +     schedule_timeout_killable(timeout)
> >> >> +
> >> >>  #define wait_event_freezable(wq, condition)                          \
> >> >>               wait_event_interruptible(wq, condition)
> >> >>
> >> >> diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
> >> >> index f8529fc..8dcfadc 100644
> >> >> --- a/net/sunrpc/sched.c
> >> >> +++ b/net/sunrpc/sched.c
> >> >> @@ -254,7 +254,7 @@ static int rpc_wait_bit_killable(void *word)
> >> >>  {
> >> >>       if (fatal_signal_pending(current))
> >> >>               return -ERESTARTSYS;
> >> >> -     freezable_schedule();
> >> >> +     freezable_schedule_unsafe();
> >> >>       return 0;
> >> >>  }
> >> >>
> >> >
> >> > Looks reasonable, but note that CIFS uses wait_event_freezekillable
> >> > with locks held too, which will likely have the same problem and will
> >> > need the same workaround for now.
> >>
> >> I didn't see any lockdep warnings reported on the mailing list when
> >> the lockdep patch was previously applied, can you point me to the lock
> >> that is held when wait_event_freezkillable is called?  I don't want to
> >> add an _unsafe call where its not needed and cause more confusion.
> >
> > It's pretty much all of the same VFS-level locks...
> >
> > Basically, when a process wants to send a synchronous SMB to a CIFS
> > server, it'll send off the request and then call wait_for_response() to
> > wait on the reply. If you need a particular call stack, then you can
> > look in the rmdir() codepath as an example:
> >
> > vfs_rmdir takes the i_mutex
> >    cifs_rmdir (via the inode ops)
> >       CIFSSMBRmDir (via the smb version ops)
> >           SendReceive
> >                wait_for_response
> >
> > ...at that point a freeze can occur while you're still holding the
> > i_mutex.
> >
> > There are many other possibilities for other codepaths that end up in
> > wait_for_response(). Once we get a solution in place for NFS, we'll
> > need to do something very similar for CIFS.
> 
> Makes sense, I will add CIFS to the patch.  Would you prefer it in the
> same or separate patches.

A new patch on top of this one would be fine with me, but I don't feel
strongly either way.

Thanks,
-- 
Jeff Layton <jlayton@xxxxxxxxxx>
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