linux.git/kernel/rcu/srcutiny.c, branch v6.18.21

srcu/tiny: Remove preempt_disable/enable() in srcu_gp_start_if_needed()

2025-09-15T12:16:28+00:00

Currently, the srcu_gp_start_if_needed() is always be invoked in
preempt disable's critical section, this commit therefore remove
redundant preempt_disable/enable() in srcu_gp_start_if_needed()
and adds a call to lockdep_assert_preemption_disabled() in order
to enable lockdep to diagnose mistaken invocations of this function
from preempts-enabled code.

Fixes: 65b4a59557f6 ("srcu: Make Tiny SRCU explicitly disable preemption")
Signed-off-by: Zqiang 
Signed-off-by: Paul E. McKenney

Merge branches 'docs.2025.02.04a', 'lazypreempt.2025.03.04a', 'misc.2025.03.04a', 'srcu.2025.02.05a' and 'torture.2025.02.05a'

2025-03-05T02:47:32+00:00

srcu: Make Tiny SRCU able to operate in preemptible kernels

2025-02-05T15:11:58+00:00

Given that SRCU allows its read-side critical sections are not just
preemptible, but also allow general blocking, there is not much
reason to restrict Tiny SRCU to non-preemptible kernels.  This commit
therefore removes Tiny SRCU dependencies on non-preemptibility, primarily
surrounding its interaction with rcutorture and early boot.

Signed-off-by: Paul E. McKenney 
Cc: Ankur Arora 
Cc: Alexei Starovoitov 
Cc: Andrii Nakryiko 
Cc: Peter Zijlstra 
Cc: Kent Overstreet 
Cc: 
Signed-off-by: Boqun Feng

rcu: rename PREEMPT_AUTO to PREEMPT_LAZY

2025-02-05T15:01:55+00:00

Replace mentions of PREEMPT_AUTO with PREEMPT_LAZY.

Also, since PREMPT_LAZY implies PREEMPTION, we can reduce the
TASKS_RCU selection criteria from this:

  NEED_TASKS_RCU && (PREEMPTION || PREEMPT_AUTO)

to this:

  NEED_TASKS_RCU && PREEMPTION

CC: Paul E. McKenney 
Reviewed-by: Frederic Weisbecker 
Reviewed-by: Sebastian Andrzej Siewior 
Signed-off-by: Ankur Arora 
Signed-off-by: Paul E. McKenney 
Signed-off-by: Boqun Feng

rcu/srcutiny: don't return before reenabling preemption

2024-11-12T20:45:20+00:00

Code after the return statement is dead. Enable preemption before
returning from srcu_drive_gp().

This will be important when/if PREEMPT_AUTO (lazy resched) gets merged.

Fixes: 65b4a59557f6 ("srcu: Make Tiny SRCU explicitly disable preemption")
Reviewed-by: Paul E. McKenney 
Signed-off-by: Michal Schmidt 
Reviewed-by: Neeraj Upadhyay 
Signed-off-by: Frederic Weisbecker

srcu: Fill out polled grace-period APIs

2024-06-18T17:13:37+00:00

This commit adds the get_completed_synchronize_srcu() and the
same_state_synchronize_srcu() functions.  The first returns a cookie
that is always interpreted as corresponding to an expired grace period.
The second does an equality comparison of a pair of cookies.

Signed-off-by: Paul E. McKenney 
Cc: Kent Overstreet

srcu: Make Tiny SRCU explicitly disable preemption

2024-04-15T09:29:48+00:00

Because Tiny SRCU is used only in kernels built with either
CONFIG_PREEMPT_NONE=y or CONFIG_PREEMPT_VOLUNTARY=y, there has not
been any need for TINY SRCU to explicitly disable preemption.  However,
the prospect of lazy preemption changes that, and the lazy-preemption
patches do result in rcutorture runs finding both too-short grace periods
and grace-period hangs for Tiny SRCU.

This commit therefore adds the needed preempt_disable() and
preempt_enable() calls to Tiny SRCU.

Signed-off-by: Paul E. McKenney 
Cc: Ankur Arora 
Cc: Thomas Gleixner 
Signed-off-by: Uladzislau Rezki (Sony)

rcu: Dump memory object info if callback function is invalid

2023-09-13T20:29:12+00:00

When a structure containing an RCU callback rhp is (incorrectly) freed
and reallocated after rhp is passed to call_rcu(), it is not unusual for
rhp->func to be set to NULL. This defeats the debugging prints used by
__call_rcu_common() in kernels built with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y,
which expect to identify the offending code using the identity of this
function.

And in kernels build without CONFIG_DEBUG_OBJECTS_RCU_HEAD=y, things
are even worse, as can be seen from this splat:

Unable to handle kernel NULL pointer dereference at virtual address 0
... ...
PC is at 0x0
LR is at rcu_do_batch+0x1c0/0x3b8
... ...
 (rcu_do_batch) from (rcu_core+0x1d4/0x284)
 (rcu_core) from (__do_softirq+0x24c/0x344)
 (__do_softirq) from (__irq_exit_rcu+0x64/0x108)
 (__irq_exit_rcu) from (irq_exit+0x8/0x10)
 (irq_exit) from (__handle_domain_irq+0x74/0x9c)
 (__handle_domain_irq) from (gic_handle_irq+0x8c/0x98)
 (gic_handle_irq) from (__irq_svc+0x5c/0x94)
 (__irq_svc) from (arch_cpu_idle+0x20/0x3c)
 (arch_cpu_idle) from (default_idle_call+0x4c/0x78)
 (default_idle_call) from (do_idle+0xf8/0x150)
 (do_idle) from (cpu_startup_entry+0x18/0x20)
 (cpu_startup_entry) from (0xc01530)

This commit therefore adds calls to mem_dump_obj(rhp) to output some
information, for example:

  slab kmalloc-256 start ffff410c45019900 pointer offset 0 size 256

This provides the rough size of the memory block and the offset of the
rcu_head structure, which as least provides at least a few clues to help
locate the problem. If the problem is reproducible, additional slab
debugging can be enabled, for example, CONFIG_DEBUG_SLAB=y, which can
provide significantly more information.

Signed-off-by: Zhen Lei 
Signed-off-by: Paul E. McKenney 
Signed-off-by: Frederic Weisbecker

rcu: Annotate SRCU's update-side lockdep dependencies

2023-03-27T18:15:59+00:00

Although all flavors of RCU readers are annotated correctly with
lockdep as recursive read locks, they do not set the lock_acquire
'check' parameter.  This means that RCU read locks are not added to
the lockdep dependency graph, which in turn means that lockdep cannot
detect RCU-based deadlocks.  This is not a problem for RCU flavors having
atomic read-side critical sections because context-based annotations can
catch these deadlocks, see for example the RCU_LOCKDEP_WARN() statement
in synchronize_rcu().  But context-based annotations are not helpful
for sleepable RCU, especially given that it is perfectly legal to do
synchronize_srcu(&srcu1) within an srcu_read_lock(&srcu2).

However, we can detect SRCU-based by: (1) Making srcu_read_lock() a
'check'ed recursive read lock and (2) Making synchronize_srcu() a empty
write lock critical section.  Even better, with the newly introduced
lock_sync(), we can avoid false positives about irq-unsafe/safe.
This commit therefore makes it so.

Note that NMI-safe SRCU read side critical sections are currently not
annotated, but might be annotated in the future.

Signed-off-by: Boqun Feng 
Signed-off-by: Paul E. McKenney 
[ boqun: Add comments for annotation per Waiman's suggestion ]
[ boqun: Fix comment warning reported by Stephen Rothwell ]
Acked-by: Peter Zijlstra (Intel) 
Signed-off-by: Boqun Feng

srcu: Make Tiny synchronize_srcu() check for readers

2022-12-01T23:49:12+00:00

This commit adds lockdep checks for illegal use of synchronize_srcu()
within same-type SRCU read-side critical sections and within normal
RCU read-side critical sections.  It also makes synchronize_srcu()
be a no-op during early boot.

These changes bring Tiny synchronize_srcu() into line with both Tree
synchronize_srcu() and Tiny synchronize_rcu().

Signed-off-by: Zqiang 
Signed-off-by: Paul E. McKenney 
Tested-by: John Ogness