[LU-16893] Fix cfs_force_sig(sig, task) deadlock on 5.3 and later kernels.

• Type: Bug
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Upstream
• Severity: 3
• Assignee: Shaun Tancheff
• Reporter: Shaun Tancheff

Description

When HAVE_FORCE_SIG_WITH_TASK is not available cfs_force_sig() is used.

However cfs_force_sig holds a spin lock on 'siglock':

#define cfs_force_sig(sig, task) \
do { \
unsigned long flags;\
\
spin_lock_irqsave(&task->sighand->siglock, flags);\
task->sighand->action[sig - 1].sa.sa_handler = SIG_DFL;\
send_sig(sig, task, 1);\
spin_unlock_irqrestore(&task->sighand->siglock, flags);         \
} while (0)

This causes a dead lock on 'siglock' as do_send_sig_info also takes the same lock.
 

cfs_force_sig(sig, task)
  -> spin_lock_irqsave(&task->sighand->siglock, flags); \
    -> send_sig(.., task,..)
     -> send_sig_info(.., task)
       -> do_send_sig_info(..., task)
         -> lock_task_sighand(task)
           -> __lock_task_sighand(task, flags)
              {
                  sighand = rcu_dereference(task->sighand);
                  spin_lock_irqsave(&sighand->siglock, *flags);
                  ...
              } 

cfs_force_sig() should release the spin lock before calling send_sig()