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u64_stats: Streamline the implementation
The u64 stats code handles 3 different cases: - 32bit UP - 32bit SMP - 64bit with an unreadable #ifdef maze, which was recently expanded with PREEMPT_RT conditionals. Reduce it to two cases (32bit and 64bit) and drop the optimization for 32bit UP as suggested by Linus. Use the new preempt_disable/enable_nested() helpers to get rid of the CONFIG_PREEMPT_RT conditionals. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lore.kernel.org/r/20220825164131.402717-9-bigeasy@linutronix.de
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‎include/linux/u64_stats_sync.h

Lines changed: 64 additions & 81 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
* Key points :
1010
*
11-
* - Use a seqcount on 32-bit SMP, only disable preemption for 32-bit UP.
11+
* - Use a seqcount on 32-bit
1212
* - The whole thing is a no-op on 64-bit architectures.
1313
*
1414
* Usage constraints:
@@ -20,7 +20,8 @@
2020
* writer and also spin forever.
2121
*
2222
* 3) Write side must use the _irqsave() variant if other writers, or a reader,
23-
* can be invoked from an IRQ context.
23+
* can be invoked from an IRQ context. On 64bit systems this variant does not
24+
* disable interrupts.
2425
*
2526
* 4) If reader fetches several counters, there is no guarantee the whole values
2627
* are consistent w.r.t. each other (remember point #2: seqcounts are not
@@ -29,11 +30,6 @@
2930
* 5) Readers are allowed to sleep or be preempted/interrupted: they perform
3031
* pure reads.
3132
*
32-
* 6) Readers must use both u64_stats_fetch_{begin,retry}_irq() if the stats
33-
* might be updated from a hardirq or softirq context (remember point #1:
34-
* seqcounts are not used for UP kernels). 32-bit UP stat readers could read
35-
* corrupted 64-bit values otherwise.
36-
*
3733
* Usage :
3834
*
3935
* Stats producer (writer) should use following template granted it already got
@@ -66,7 +62,7 @@
6662
#include <linux/seqlock.h>
6763

6864
struct u64_stats_sync {
69-
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
65+
#if BITS_PER_LONG == 32
7066
seqcount_t seq;
7167
#endif
7268
};
@@ -98,7 +94,22 @@ static inline void u64_stats_inc(u64_stats_t *p)
9894
local64_inc(&p->v);
9995
}
10096

101-
#else
97+
static inline void u64_stats_init(struct u64_stats_sync *syncp) { }
98+
static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp) { }
99+
static inline void __u64_stats_update_end(struct u64_stats_sync *syncp) { }
100+
static inline unsigned long __u64_stats_irqsave(void) { return 0; }
101+
static inline void __u64_stats_irqrestore(unsigned long flags) { }
102+
static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
103+
{
104+
return 0;
105+
}
106+
static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
107+
unsigned int start)
108+
{
109+
return false;
110+
}
111+
112+
#else /* 64 bit */
102113

103114
typedef struct {
104115
u64 v;
@@ -123,123 +134,95 @@ static inline void u64_stats_inc(u64_stats_t *p)
123134
{
124135
p->v++;
125136
}
126-
#endif
127137

128-
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
129-
#define u64_stats_init(syncp) seqcount_init(&(syncp)->seq)
130-
#else
131138
static inline void u64_stats_init(struct u64_stats_sync *syncp)
132139
{
140+
seqcount_init(&syncp->seq);
133141
}
134-
#endif
135142

136-
static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
143+
static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp)
137144
{
138-
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
139-
if (IS_ENABLED(CONFIG_PREEMPT_RT))
140-
preempt_disable();
145+
preempt_disable_nested();
141146
write_seqcount_begin(&syncp->seq);
142-
#endif
143147
}
144148

145-
static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
149+
static inline void __u64_stats_update_end(struct u64_stats_sync *syncp)
146150
{
147-
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
148151
write_seqcount_end(&syncp->seq);
149-
if (IS_ENABLED(CONFIG_PREEMPT_RT))
150-
preempt_enable();
151-
#endif
152+
preempt_enable_nested();
152153
}
153154

154-
static inline unsigned long
155-
u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
155+
static inline unsigned long __u64_stats_irqsave(void)
156156
{
157-
unsigned long flags = 0;
157+
unsigned long flags;
158158

159-
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
160-
if (IS_ENABLED(CONFIG_PREEMPT_RT))
161-
preempt_disable();
162-
else
163-
local_irq_save(flags);
164-
write_seqcount_begin(&syncp->seq);
165-
#endif
159+
local_irq_save(flags);
166160
return flags;
167161
}
168162

169-
static inline void
170-
u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
171-
unsigned long flags)
163+
static inline void __u64_stats_irqrestore(unsigned long flags)
172164
{
173-
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
174-
write_seqcount_end(&syncp->seq);
175-
if (IS_ENABLED(CONFIG_PREEMPT_RT))
176-
preempt_enable();
177-
else
178-
local_irq_restore(flags);
179-
#endif
165+
local_irq_restore(flags);
180166
}
181167

182168
static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
183169
{
184-
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
185170
return read_seqcount_begin(&syncp->seq);
186-
#else
187-
return 0;
188-
#endif
189171
}
190172

191-
static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
173+
static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
174+
unsigned int start)
192175
{
193-
#if BITS_PER_LONG == 32 && (!defined(CONFIG_SMP) && !defined(CONFIG_PREEMPT_RT))
194-
preempt_disable();
195-
#endif
196-
return __u64_stats_fetch_begin(syncp);
176+
return read_seqcount_retry(&syncp->seq, start);
197177
}
178+
#endif /* !64 bit */
198179

199-
static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
200-
unsigned int start)
180+
static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
201181
{
202-
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
203-
return read_seqcount_retry(&syncp->seq, start);
204-
#else
205-
return false;
206-
#endif
182+
__u64_stats_update_begin(syncp);
183+
}
184+
185+
static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
186+
{
187+
__u64_stats_update_end(syncp);
188+
}
189+
190+
static inline unsigned long u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
191+
{
192+
unsigned long flags = __u64_stats_irqsave();
193+
194+
__u64_stats_update_begin(syncp);
195+
return flags;
196+
}
197+
198+
static inline void u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
199+
unsigned long flags)
200+
{
201+
__u64_stats_update_end(syncp);
202+
__u64_stats_irqrestore(flags);
203+
}
204+
205+
static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
206+
{
207+
return __u64_stats_fetch_begin(syncp);
207208
}
208209

209210
static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
210211
unsigned int start)
211212
{
212-
#if BITS_PER_LONG == 32 && (!defined(CONFIG_SMP) && !defined(CONFIG_PREEMPT_RT))
213-
preempt_enable();
214-
#endif
215213
return __u64_stats_fetch_retry(syncp, start);
216214
}
217215

218-
/*
219-
* In case irq handlers can update u64 counters, readers can use following helpers
220-
* - SMP 32bit arches use seqcount protection, irq safe.
221-
* - UP 32bit must disable irqs.
222-
* - 64bit have no problem atomically reading u64 values, irq safe.
223-
*/
216+
/* Obsolete interfaces */
224217
static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
225218
{
226-
#if BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT_RT)
227-
preempt_disable();
228-
#elif BITS_PER_LONG == 32 && !defined(CONFIG_SMP)
229-
local_irq_disable();
230-
#endif
231-
return __u64_stats_fetch_begin(syncp);
219+
return u64_stats_fetch_begin(syncp);
232220
}
233221

234222
static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
235223
unsigned int start)
236224
{
237-
#if BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT_RT)
238-
preempt_enable();
239-
#elif BITS_PER_LONG == 32 && !defined(CONFIG_SMP)
240-
local_irq_enable();
241-
#endif
242-
return __u64_stats_fetch_retry(syncp, start);
225+
return u64_stats_fetch_retry(syncp, start);
243226
}
244227

245228
#endif /* _LINUX_U64_STATS_SYNC_H */

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