Messages in this thread | | | Date | Thu, 01 Mar 2007 07:11:30 -0500 | From | Prarit Bhargava <> | Subject | Re: [PATCH]: Use stop_machine_run in the Intel RNG driver |
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> I think what you're describing here is just the standard old > smp_call_function() deadlock, rather than anything which is specific to > intel-rng, yes? > > It is "well known" that you can't call smp_call_function() with local > interrupts disabled. In fact i386 will spit a warning if you try it. > > > intel-rng doesn't do that, but what it _does_ do is: > > smp_call_function(..., wait = 0); > local_irq_disable(); > > so some CPUs will still be entering the IPI while this CPU has gone and > disabled interrupts, thus exposing us to the deadlock, yes? >
Not quite Andrew. This was a different and little more complicated.
The deadlock occurs because two CPUs are in contention over a rw_lock and the "call_function" puts the CPUs in such a state that no forward progress will be made until the calling CPU has completed it's code.
Here's a more detailed example (sorry for the cut-and-paste):
1. CPU A has done read_lock(&lock), and has acquired the lock. 2. CPU B has done write_lock_irq(&lock) and is waiting for A to release the lock. CPU B has disabled interrupts while waiting for the interrupt:
void __lockfunc _write_lock_irq(rwlock_t *lock) { local_irq_disable(); preempt_disable(); rwlock_acquire(&lock->dep_map, 0, 0, _RET_IP_); _raw_write_lock(lock); }
3. CPU C issues smp_call_function, as in the case of the intel-rng driver:
set_mb(waitflag, 1); smp_call_function(intel_init_wait, NULL, 1, 0); ... // do some stuff with interrupts disabled ... set_mb(waitflag, 0);
where
static char __initdata waitflag;
static void __init intel_init_wait(void *unused) { while (waitflag) cpu_relax(); }
In this code the calling processor, C, has issued an IPI and disabled interrupts on every processor except itself. When each processor takes the IPI it runs intel_init_wait and waits in a tight loop until waitflag is zero. ie) no forward progress on any CPU.
CPU C will not execute the code below the smp_call_function until all processors have started (not completed!) the IPI function. From call_smp_function:
cpus = num_online_cpus() - 1; ...
/* Send a message to all other CPUs and wait for them to respond */ send_IPI_allbutself(CALL_FUNCTION_VECTOR);
/* Wait for response */ while (atomic_read(&data.started) != cpus) cpu_relax();
So CPU C is waiting here.
4. CPU A, which holds the lock sees the IPI and is in the intel_init_wait code, happily waiting. CPU A has incremented data.started. CPU A will stay in this loop until CPU C sets waitflag = 0.
5. CPU B, if you recall is _waiting with interrupts disabled_ for CPU A to release the lock. It does not see the IPI because it has interrupts disabled. It will not see the IPI until CPU A has released the lock.
6. CPU C is eventually only waiting for CPU B to do the final increment of data.started = cpus. CPU B is waiting for CPU A to release the lock. CPU A is executing a tight loop which it will not exit from until CPU C can set waitflag to zero.
That's a 3-way deadlock.
So, the issue is placing the other CPUs in a state that they do not make forward progress. The deadlock occurs before the calling CPU has disabled interrupts in the code in step 3.
I also tested this code without the __init tags and explicitly coding waitflag=0 to avoid the gcc only setting .bss section variables to zero error that someone fixed last week. I also removed the code that disabled interrupts on the calling processor which had no effect -- at first I thought it was a simple interrupt issue ...
Maybe smp_call_function needs a written warning that the called function should not "suspend" CPUs?
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