On Thu, 7 Mar 2024, Paul E. McKenney wrote:
> On Thu, Mar 07, 2024 at 12:00:44PM -0800, Linus Torvalds wrote:
> > On Thu, 7 Mar 2024 at 11:47, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > >
> > > > - The per-thread counter (Thread-Local Storage) incremented by a single
> > > > thread, read by various threads concurrently, is a good target
> > > > for WRITE_ONCE()/READ_ONCE() pairing. This is actually what we do in
> > > > various liburcu implementations which track read-side critical sections
> > > > per-thread.
> > >
> > > Agreed, but do any of these use WRITE_ONCE(x, READ_ONCE(x) + 1) or
> > > similar?
> >
> > Absolutely not.
> >
> > The READ_ONCE->WRITE_ONCE pattern is almost certainly a bug.
> >
> > The valid reason to have a WRITE_ONCE() is that there's a _later_
> > READ_ONCE() on another CPU.
> >
> > So WRITE_ONCE->READ_ONCE (across threads) is very valid. But
> > READ_ONCE->WRITE_ONCE (inside a thread) simply is not a valid
> > operation.
> >
> > We do have things like "local_t", which allows for non-smp-safe local
> > thread atomic accesses, but they explicitly are *NOT* about some kind
> > of READ_ONCE -> WRITE_ONCE sequence that by definition cannot be
> > atomic unless you disable interrupts and disable preemption (at which
> > point they become pointless and only generate worse code).
> >
> > But the point of "local_t" is that you can do things that aresafe if
> > there is no SMP issues. They are kind of an extension of the
> > percpu_add() kind of operations.
> >
> > In fact, I think it might be interesting to catch those
> > READ_ONCE->WRITE_ONCE chains (perhaps with coccinelle?) because they
> > are a sign of bugs.
>
> Good point, adding Julia Lawall on CC.
I tried the following:
@@
expression x;
@@
*WRITE_ONCE(x,<+...READ_ONCE(x)...+>)
This gave a number of results, shown below. Let me know if some of them
are undesirable.
I also tried:
@@
expression x,v;
*v = READ_ONCE(x)
... when != v
*WRITE_ONCE(x,<+...v...+>)
I found the results somewhat less compelling. Maybe the following are
interesting (note that - means this line is interesting, not a suggestion
to remove it):
479 files match
diff -u -p /home/julia/linux/net/netfilter/nf_tables_api.c /tmp/nothing/net/netfilter/nf_tables_api.c
--- /home/julia/linux/net/netfilter/nf_tables_api.c
+++ /tmp/nothing/net/netfilter/nf_tables_api.c
@@ -10026,8 +10026,6 @@ static unsigned int nft_gc_seq_begin(str
unsigned int gc_seq;
/* Bump gc counter, it becomes odd, this is the busy mark. */
- gc_seq = READ_ONCE(nft_net->gc_seq);
- WRITE_ONCE(nft_net->gc_seq, ++gc_seq);
return gc_seq;
}
diff -u -p /home/julia/linux/fs/xfs/xfs_icache.c /tmp/nothing/fs/xfs/xfs_icache.c
--- /home/julia/linux/fs/xfs/xfs_icache.c
+++ /tmp/nothing/fs/xfs/xfs_icache.c
@@ -2076,8 +2076,6 @@ xfs_inodegc_queue(
cpu_nr = get_cpu();
gc = this_cpu_ptr(mp->m_inodegc);
llist_add(&ip->i_gclist, &gc->list);
- items = READ_ONCE(gc->items);
- WRITE_ONCE(gc->items, items + 1);
shrinker_hits = READ_ONCE(gc->shrinker_hits);
/*
@@ -2199,9 +2197,7 @@ xfs_inodegc_shrinker_scan(
for_each_cpu(cpu, &mp->m_inodegc_cpumask) {
gc = per_cpu_ptr(mp->m_inodegc, cpu);
if (!llist_empty(&gc->list)) {
- unsigned int h = READ_ONCE(gc->shrinker_hits);
- WRITE_ONCE(gc->shrinker_hits, h + 1);
mod_delayed_work_on(cpu, mp->m_inodegc_wq, &gc->work, 0);
no_items = false;
}
In other cases, more work is done between the read and the write. I can send themif of interest.
julia
diff -u -p /home/julia/linux/kernel/sched/fair.c /tmp/nothing/kernel/sched/fair.c
--- /home/julia/linux/kernel/sched/fair.c
+++ /tmp/nothing/kernel/sched/fair.c
@@ -3153,7 +3153,6 @@ static void reset_ptenuma_scan(struct ta
* statistical sampling. Use READ_ONCE/WRITE_ONCE, which are not
* expensive, to avoid any form of compiler optimizations:
*/
- WRITE_ONCE(p->mm->numa_scan_seq, READ_ONCE(p->mm->numa_scan_seq) + 1);
p->mm->numa_scan_offset = 0;
}
diff -u -p /home/julia/linux/net/ipv4/inet_hashtables.c /tmp/nothing/net/ipv4/inet_hashtables.c
--- /home/julia/linux/net/ipv4/inet_hashtables.c
+++ /tmp/nothing/net/ipv4/inet_hashtables.c
@@ -1109,7 +1109,6 @@ ok:
* it may be inexistent.
*/
i = max_t(int, i, get_random_u32_below(8) * step);
- WRITE_ONCE(table_perturb[index], READ_ONCE(table_perturb[index]) + i + step);
/* Head lock still held and bh's disabled */
inet_bind_hash(sk, tb, tb2, port);
diff -u -p /home/julia/linux/kernel/rcu/tree.c /tmp/nothing/kernel/rcu/tree.c
--- /home/julia/linux/kernel/rcu/tree.c
+++ /tmp/nothing/kernel/rcu/tree.c
@@ -1620,8 +1620,6 @@ static void rcu_gp_fqs(bool first_time)
/* Clear flag to prevent immediate re-entry. */
if (READ_ONCE(rcu_state.gp_flags) & RCU_GP_FLAG_FQS) {
raw_spin_lock_irq_rcu_node(rnp);
- WRITE_ONCE(rcu_state.gp_flags,
- READ_ONCE(rcu_state.gp_flags) & ~RCU_GP_FLAG_FQS);
raw_spin_unlock_irq_rcu_node(rnp);
}
}
@@ -1882,8 +1880,6 @@ static void rcu_report_qs_rsp(unsigned l
{
raw_lockdep_assert_held_rcu_node(rcu_get_root());
WARN_ON_ONCE(!rcu_gp_in_progress());
- WRITE_ONCE(rcu_state.gp_flags,
- READ_ONCE(rcu_state.gp_flags) | RCU_GP_FLAG_FQS);
raw_spin_unlock_irqrestore_rcu_node(rcu_get_root(), flags);
rcu_gp_kthread_wake();
}
@@ -2392,8 +2388,6 @@ void rcu_force_quiescent_state(void)
raw_spin_unlock_irqrestore_rcu_node(rnp_old, flags);
return; /* Someone beat us to it. */
}
- WRITE_ONCE(rcu_state.gp_flags,
- READ_ONCE(rcu_state.gp_flags) | RCU_GP_FLAG_FQS);
raw_spin_unlock_irqrestore_rcu_node(rnp_old, flags);
rcu_gp_kthread_wake();
}
diff -u -p /home/julia/linux/drivers/net/ethernet/cavium/liquidio/cn23xx_vf_device.c /tmp/nothing/drivers/net/ethernet/cavium/liquidio/cn23xx_vf_device.c
--- /home/julia/linux/drivers/net/ethernet/cavium/liquidio/cn23xx_vf_device.c
+++ /tmp/nothing/drivers/net/ethernet/cavium/liquidio/cn23xx_vf_device.c
@@ -80,8 +80,6 @@ static int cn23xx_vf_reset_io_queues(str
q_no);
return -1;
}
- WRITE_ONCE(reg_val, READ_ONCE(reg_val) &
- ~CN23XX_PKT_INPUT_CTL_RST);
octeon_write_csr64(oct, CN23XX_VF_SLI_IQ_PKT_CONTROL64(q_no),
READ_ONCE(reg_val));
diff -u -p /home/julia/linux/kernel/rcu/srcutree.c /tmp/nothing/kernel/rcu/srcutree.c
--- /home/julia/linux/kernel/rcu/srcutree.c
+++ /tmp/nothing/kernel/rcu/srcutree.c
@@ -628,7 +628,6 @@ static unsigned long srcu_get_delay(stru
if (time_after(j, gpstart))
jbase += j - gpstart;
if (!jbase) {
- WRITE_ONCE(sup->srcu_n_exp_nodelay, READ_ONCE(sup->srcu_n_exp_nodelay) + 1);
if (READ_ONCE(sup->srcu_n_exp_nodelay) > srcu_max_nodelay_phase)
jbase = 1;
}
@@ -1799,7 +1798,6 @@ static void process_srcu(struct work_str
} else {
j = jiffies;
if (READ_ONCE(sup->reschedule_jiffies) == j) {
- WRITE_ONCE(sup->reschedule_count, READ_ONCE(sup->reschedule_count) + 1);
if (READ_ONCE(sup->reschedule_count) > srcu_max_nodelay)
curdelay = 1;
} else {
diff -u -p /home/julia/linux/kernel/kcsan/kcsan_test.c /tmp/nothing/kernel/kcsan/kcsan_test.c
--- /home/julia/linux/kernel/kcsan/kcsan_test.c
+++ /tmp/nothing/kernel/kcsan/kcsan_test.c
@@ -381,7 +381,6 @@ static noinline void test_kernel_change_
test_var ^= TEST_CHANGE_BITS;
kcsan_nestable_atomic_end();
} else
- WRITE_ONCE(test_var, READ_ONCE(test_var) ^ TEST_CHANGE_BITS);
}
static noinline void test_kernel_assert_bits_change(void)
diff -u -p /home/julia/linux/arch/s390/kernel/idle.c /tmp/nothing/arch/s390/kernel/idle.c
--- /home/julia/linux/arch/s390/kernel/idle.c
+++ /tmp/nothing/arch/s390/kernel/idle.c
@@ -43,8 +43,6 @@ void account_idle_time_irq(void)
S390_lowcore.last_update_timer = S390_lowcore.sys_enter_timer;
/* Account time spent with enabled wait psw loaded as idle time. */
- WRITE_ONCE(idle->idle_time, READ_ONCE(idle->idle_time) + idle_time);
- WRITE_ONCE(idle->idle_count, READ_ONCE(idle->idle_count) + 1);
account_idle_time(cputime_to_nsecs(idle_time));
}
diff -u -p /home/julia/linux/mm/mmap.c /tmp/nothing/mm/mmap.c
--- /home/julia/linux/mm/mmap.c
+++ /tmp/nothing/mm/mmap.c
@@ -3476,7 +3476,6 @@ bool may_expand_vm(struct mm_struct *mm,
void vm_stat_account(struct mm_struct *mm, vm_flags_t flags, long npages)
{
- WRITE_ONCE(mm->total_vm, READ_ONCE(mm->total_vm)+npages);
if (is_exec_mapping(flags))
mm->exec_vm += npages;
diff -u -p /home/julia/linux/fs/xfs/libxfs/xfs_iext_tree.c /tmp/nothing/fs/xfs/libxfs/xfs_iext_tree.c
--- /home/julia/linux/fs/xfs/libxfs/xfs_iext_tree.c
+++ /tmp/nothing/fs/xfs/libxfs/xfs_iext_tree.c
@@ -618,7 +618,6 @@ xfs_iext_realloc_root(
*/
static inline void xfs_iext_inc_seq(struct xfs_ifork *ifp)
{
- WRITE_ONCE(ifp->if_seq, READ_ONCE(ifp->if_seq) + 1);
}
void
diff -u -p /home/julia/linux/drivers/net/ethernet/cavium/liquidio/cn23xx_pf_device.c /tmp/nothing/drivers/net/ethernet/cavium/liquidio/cn23xx_pf_device.c
--- /home/julia/linux/drivers/net/ethernet/cavium/liquidio/cn23xx_pf_device.c
+++ /tmp/nothing/drivers/net/ethernet/cavium/liquidio/cn23xx_pf_device.c
@@ -379,8 +379,6 @@ static int cn23xx_reset_io_queues(struct
q_no);
return -1;
}
- WRITE_ONCE(reg_val, READ_ONCE(reg_val) &
- ~CN23XX_PKT_INPUT_CTL_RST);
octeon_write_csr64(oct, CN23XX_SLI_IQ_PKT_CONTROL64(q_no),
READ_ONCE(reg_val));
@@ -879,9 +877,6 @@ static void cn23xx_disable_io_queues(str
/* start the Reset for a particular ring */
WRITE_ONCE(d64, octeon_read_csr64(
oct, CN23XX_SLI_IQ_PKT_CONTROL64(q_no)));
- WRITE_ONCE(d64, READ_ONCE(d64) &
- (~(CN23XX_PKT_INPUT_CTL_RING_ENB)));
- WRITE_ONCE(d64, READ_ONCE(d64) | CN23XX_PKT_INPUT_CTL_RST);
octeon_write_csr64(oct, CN23XX_SLI_IQ_PKT_CONTROL64(q_no),
READ_ONCE(d64));
diff -u -p /home/julia/linux/mm/kfence/kfence_test.c /tmp/nothing/mm/kfence/kfence_test.c
--- /home/julia/linux/mm/kfence/kfence_test.c
+++ /tmp/nothing/mm/kfence/kfence_test.c
@@ -501,7 +501,6 @@ static void test_kmalloc_aligned_oob_wri
* fault immediately after it.
*/
expect.addr = buf + size;
- WRITE_ONCE(*expect.addr, READ_ONCE(*expect.addr) + 1);
KUNIT_EXPECT_FALSE(test, report_available());
test_free(buf);
KUNIT_EXPECT_TRUE(test, report_matches(&expect));
diff -u -p /home/julia/linux/io_uring/io_uring.c /tmp/nothing/io_uring/io_uring.c
--- /home/julia/linux/io_uring/io_uring.c
+++ /tmp/nothing/io_uring/io_uring.c
@@ -363,7 +363,6 @@ static void io_account_cq_overflow(struc
{
struct io_rings *r = ctx->rings;
- WRITE_ONCE(r->cq_overflow, READ_ONCE(r->cq_overflow) + 1);
ctx->cq_extra--;
}
@@ -2403,8 +2402,6 @@ static bool io_get_sqe(struct io_ring_ct
spin_lock(&ctx->completion_lock);
ctx->cq_extra--;
spin_unlock(&ctx->completion_lock);
- WRITE_ONCE(ctx->rings->sq_dropped,
- READ_ONCE(ctx->rings->sq_dropped) + 1);
return false;
}
}
diff -u -p /home/julia/linux/security/apparmor/apparmorfs.c /tmp/nothing/security/apparmor/apparmorfs.c
--- /home/julia/linux/security/apparmor/apparmorfs.c
+++ /tmp/nothing/security/apparmor/apparmorfs.c
@@ -596,7 +596,6 @@ static __poll_t ns_revision_poll(struct
void __aa_bump_ns_revision(struct aa_ns *ns)
{
- WRITE_ONCE(ns->revision, READ_ONCE(ns->revision) + 1);
wake_up_interruptible(&ns->wait);
}
diff -u -p /home/julia/linux/arch/riscv/kvm/vmid.c /tmp/nothing/arch/riscv/kvm/vmid.c
--- /home/julia/linux/arch/riscv/kvm/vmid.c
+++ /tmp/nothing/arch/riscv/kvm/vmid.c
@@ -90,7 +90,6 @@ void kvm_riscv_gstage_vmid_update(struct
/* First user of a new VMID version? */
if (unlikely(vmid_next == 0)) {
- WRITE_ONCE(vmid_version, READ_ONCE(vmid_version) + 1);
vmid_next = 1;
/*
